Methods for the obtainment of articles of chromium or containing chromium and in articles obtained by these methods



Sept. 11, 1962 P. GALMICHE 3,

METHODS FOR THE OBTAINMENT OF ARTICLES OF CHROMIUM OR CONTAINING CHROMIUM AND IN ARTICLES OBTAINED BY THESE METHODS 2 Sheets-Sheet 1 Filed March 31, 1958 /NVENTOR Philipp Galm'mke i.

ATTORNEYS 3,053,649 F CHROMIUM 2 Sheets-Sheet 2 P. GALMICHE OBTAINED BY THESE METHODS METHODS FOR THE OBTAINMENT 0F ARTICLES 0 OR CONTAINING CHROMIUM AND IN ARTICLES INVENTOR Phi/i p)" CrAbM/CIIE B CWTWM ATTORNEYS Sept. 11,

Filed March :51, 1958 3,053,649 Patented Sept. 11, 1952 3,053,649 METHODS FOR THE OBTAINMENT F ARTICLES 0F CHROMIUM 0R CONTAINING CHROJH 31;? IN ARTICLES OBTAINED BY TIESE NETH- Philippe Galmiche, Paris, France, assignor to Oiiice National dEtutles et de Recherches Aeronautiques O.N.E.R.A., Chatillon-sous-Bagneau (Seine), France, a corporation of France Filed Mar. 31, 1958, Ser. No. 725,066 Claims priority, application France Apr. 2, 1957 20 Claims. (Cl. 75-84) The present invention relates to the treatment of chromium, either alone or mixed or alloyed with at least one other metal, for instance one metal of the group consisting of nickel, cobalt, iron, molybdenum, tungsten and titanium.

Chromium, such as it is available industrially at the present time and whatever be the process by means of which it is obtained (for instance alumino-thermy, silicothermy, magneso-thermy or electrolysis), and alloys containing a high amount of such chromium, are brittle and lacking in ductility. It is generally considered that these defects are due to the presence in such materials of various impurities the most important of which are oxygen, nitrogen, carbon, sulfur, silicon, aluminium. This has been a serious drawback for many uses of such materials and in particular when it was desired to manufacture sintered pieces of chromium or chromium alloys containing a substantial amount of chromium as above referred to.

The chief object of the present invention is to provide a method of treating chromium and chromium alloys which is tree from the above mentioned defects and in particular which gives products having remarkable qualities of plasticity and ductility.

For this purpose, according to this invention, use is made of a starting material at least partly constituted by ordinary or commercial chromium and consisting of a multiplicity of grains of an average size of at most onetenth of a millimeter which are juxtaposed to form a porous mass after which said mass is heated at a temperature of at least 600 C. while there is constantly present therein, around said grains, a gaseous mixture of vapors of at least a halide of one metal contained in said mass and of an amount of hydrogen sufiiciently low not to reduce the whole of said halide, the duration and temperature of this heating being chosen, in view of the conditions of operation, to achieve chromium purification before any substantial sintering of said mass has been able to begin, and said mass is protected, during and after said heating, against lasting contact with any gaseous medium containing material amounts of at least one of the gases of the group consisting of oxygen, water vapor, nitrogen and ammonia. The duration of the above mentioned heating is, other conditions being the same, the shorter as the temperature thereof is higher.

In the above recitation of the invention, the Word juxtaposed is taken to mean placed in nearness or contiguity. The grains are placed in contiguity when the mass above mentioned is constituted exclusively by grains of chromium or a chromium alloy. On the contrary when said mass contains, in addition to such grains, another substance, also in the form of fine grains, interposed between the first mentioned grains, said first mentioned grains are still juxtaposed because they are placed in nearness to one another.

As for the term mass as used hereinabove, it is meant to designate either a powder made of fine grains freely movable with respect to one another or an agglomerate in which the grains are fixed with respect to one an- 2 other, such an agglomerate being obtained from such a powder, for instance by mechanical pressing and/or by incorporating therein a suitable binder which is subsequently eliminated so that the mass is suitably porous.

It is therefore a characteristic of this invention that the chromium to be treated is, at least during one step of its treatment, in the form of powder.

It should be understood that a substantial sintering is one corresponding to a porosity reduced to about 5% in the case of a chromium alloy and about 10% in the case of pure chromium, the temperature of heating being 900 C. in the case of pure chromium and the halide being in both cases a fluoride. These porosities are greater when other halides are used. They are higher in the case of chlorine, still higher in the case of bromine and still higher in the case of iodine.

Finally, it is desired to define what is meant by material amounts of at least one of the gases of the group consisting of oxygen, water vapor, nitrogen and ammonia. Such a material amount is one which, if present in the atmosphere of the container where the treatment is performed, would be capable of preventing the elimination of the above mentioned impurities by reversing the direction in which takes place the reversible reactions of purification.

According to another feature of the invention, in order to obtain ductile chromium in the state of powder, the starting material is constituted by a powder of chro mum, or an alloy rich in chromium, mixed with an inert diluting substance in the form of a fine powder, said diluting substance being capable of being eliminated at the end of the treatment, after cooling of the product, and consisting of a metallic oxide the metal of which does not diffuse into chromium and the heat of formation of which is higher than that of chromium oxide. Magnesia is a preferred diluting substance according to the present invention.

According to still another feature of the invention, in order to improve the elimination of oxygen from the starting material, said starting material is mixed with a small amount (averaging some thousandths by weight in the form of turnings or powder) of magnesium and/or at least one metal having an oxide the heat of formation of which is higher than that of chromium oxide, such for instance as calcium, thorium or zirconium.

Preferred forms of this invention will be hereinafter described with reference to the appended drawings given merely by way of example and in which:

FIGS. 1 and 2 show apparatus 'for carnying out the method according to the invention.

FIG. 3 is a curve illustrating the properties of a chromium powder obtained according to the invention.

In order to obtain the desired presence, in the mass above referred to, of a gaseous mixture of vapors of a halide of at least one metal contained in said mass and of a suitably limited amount of hydrogen it is possible, within the scope of the invention, to proceed in various ways, and in particular as follows.

According to one form of the invention, there is incorporated in the mass a small amount, for instance from 0.1 to 3 percent, of a little volatile halide of at least one of the metals present in said mass and the whole is heated, at temperatures ranging firom 600 C. to 1,400 C., in a stream of highly purified hydrogen flowing at a slow rate.

It should be understood that the incorporation of the above mentioned little volatile metal halide in the mass may be obtained by adding another halide or a halogen, for instance iodine, into the mass, the desired metal halide being formed in situ by reaction of said other halide or of said halogen with the metal of the mass.

in the present specification and claims, the term little volatile halide means that the volatilizati-on of such a halide at the temperature of treatment is small so that the disengagement of vapors of said halide at said temperature takes place but gradually.

Instead of placing the mass to be treated directly in a stream of purified hydrogen, it is preferred to place it in a treatment box which is only partly gastight so as to permit a limited exchange of gases between the inside of the box and the outside thereof, and to heat said box, at the above mentioned temperatures, in a stream of hydrogen circulating at a low rate of flow (some cubic centimeters per second) along said box. Thus hydrogen from said stream can enter said box, since it is not quite gastight and penetrate into the mass to be treated.

As for the halide vapors which are to be present in the porous mass in treatment they are advantageously obtained, in this case also, by incorporating in said mass either said halide itself or a substance (or substances) capable of forming it in situ at the temperature of treatment.

Advantageously, the treatment box further contains a little volatile halide (or a mixture of substances which, at the temperature of treatment, react upon one another to form said halide) so as to maintain inside said box an atmosphere of said halide.

-In some cases, this last mentioned halide is a halide of one of the metals contained in the mass and it may constitute the means, or one of the means, for having vapors of said halide present in the porous mass.

The hydrogen that circulates along said box is advantageously purified but it may contain an amount of oxygen or water vapors not exceeding tenpercent. But, except when the mass to be treated is of a composition corresponding to that of stainless steel, this hydrogen must containing as little nitrogen and/or ammonia as possible. For this purpose, the stream of hydrogen circulating along the treatment box is first made to pass upon grains of titanium oxide at a temperature ranging from 700 C. to 1,000 C.

iFIGS. 1 and 2 illustrate the method above described.

On FIG. 1, which shows an apparatus suitable for treatments at temperatures above 1,300 C., the treatment box is designated by reference numeral 5. The masses to be treated, designated by reference numeral 7, are carried by a support 6. A mass 9 capable of giving off vapors of at least one halide or halogen is provided at the bottom of the box. Said mass 9 is for instance a mixture of chromium and chromium fluoride obtained from a mixture of chromium and ammonium fluoride previously heated at 250300 C. so as to drive off nitrogen. This preliminary heating is unnecessary if the product to be obtained from the mass in treatment is stainless steel, because in this case the presence of some nitrogen is not objectionable.

Box is placed in a tube 2 through which a stream of hydrogen entering at is made to circulate at a relatively low flow rate as above mentioned. This tube, made of carbon, is surrounded by an electric resistance 1 for heating the whole to the desired temperature.

Box 5 is only partly gastight due to the fact that its cover 5,, is mounted thereon in such manner as to permit gaseous exchanges between the inside of the box and the stream of hydrogen flowing from tube 2.

As an example of such a box only partly gastight that was used according to this invention, the following indications may be given. Such a box was cylindrical. Its cover fitted with an easy fit, that is to say with a play ranging from 2 to 3 tenths of a millimeter. It rested on the edge of the box and extended over millimeters in the axial direction. The diameter of said box was 150 mms. and its height 200 mms.

The construction of FIG. 2 is similar to that of FIG. 1 with the difference that the tube 3 through which hydrogen circulates is horizontal and resistance 4 is a Kantahl resistance. This apparatus is suitable for treatments at temperatures below 1,300" C.

Concerning the materials of which are to be made the boxes 5 and the supports 6 located therein, if the treatment temperature is higher than 1,300 C., said parts should be made of molybdenum or of tungsten. These metals are suitable for this purpose although they are attacked by halogen acids, and in particular hydrofluoric acid, and the molybdenum and the tungsten halides, and in particular fluorides, are very volatile. But if this attack is taking place, molybdenum or tungsten is superficially chromized, which stops the attack, and the halide produced by the attack is immediately reduced by hydrogen so as to reconstitute the initial metal.

However, as this chromizing renders these metals brittle, it may be advantageous to, make use of boxes made of alumina or carbon with only an inner lining of molybdenum.

When masses 7 are treated at temperatures below 1,300 C., the treatment boxes or at least the inner wall thereof (which is in the presence of the. treatment atmosphere) may be made of iron, stainless steel or a nickel or a cobalt alloy.

Nickel, used as a. material to constitute the treatment box, has the advantage of being permeable only to hydrogen while preventing the passage of oxygen and nitrogen therethrough.

Some indications will now be given concerning the various impurities present in chromium and how they are eliminated by the method of this invention.

Oxygen is present in the masses 7 to be treated in the form of oxides and in particular of chromium oxide. These oxides are reduced by the hydrogen allowed to enter box 5 owing to the presence in said box of halide vapors, fluoride vapors being the most active for this purpose.

Account must however be taken of the fact that hydrogen, even purified, generally contains some small amounts of oxygen. This oxygen will therefore enter box 5 continuously through the passage between cover 5,, and the box itself. As the inflow of too high a proportion of oxygen into the box is obviously objectionable, the hydrogen stream surrounding the box should therefore contain as little oxygen as possible.

Furthermore, in order to prevent, as far as possible, the oxygen present in the hydrogen stream from entering the box through the leak between cover 5,, and the box itself, there is provided, inside the box and just under cover 5,,, a screen 8 (which in FIG. 1 is constituted by a juxtaposition of small lumps of chromium carried by a perforated support and in FIG. 2. is constituted by a porous plate 8 of chromium). The oxygen entering box 5 reacts with this chromium screen which it oxidizes and is thus prevented from reaching masses 7.

But it should be understood that, whereas there would be no remedy to the continuous inflow of too great an amount of oxygen into the box, a temporary increase of the amount of oxygen present in the box, such as would be produced by an increased disengagement of oxygen from an oxide present in the box, and for instance included in the mass in treatment, may be taken care of by the hydrogen fed into the box from the outside owing to the favorable influence of the halide vapors present in the box, especially when said halide is a fluoride.

Of course, every care must be taken to avoid the presence of oxygen sources in the treatment box 5.

Such sources might be constituted by the supports of masses 7 in box 5, because it is customary to use refractory oxides to constitute supports for pieces subjected to heating at relatively high temperatures. For instance if masses 7 are agglomerate blocks of low mechanical resistance it may be necessary to have them supported by a bed of refractory oxide in the powder form. In this case, said refractory oxide must be one having a heat of formation as high as possible (for instance Mg-O, ThO TiO It must not be attacked by the halide present in the box and it must not be capable of sinter-ing therewith. If masses 7 are in the form of a powder, they are placed in containers, generally made of sintered refractory oxides. But in this case the heat of formation of said oxides may be close to that of chromium oxide (for instance zirconium oxide may be used). This is because the reactive area of such a container is negligible as compared with that of a powder.

In some cases and for instance when the amount of powder to be treated is important and/or when said powder contains initially a great quantity of oxide (case of electrolytic chromium powder), the elimination of oxygen by reduction in the presence of a halide may be insuflicient. Furthermore when the powder contains a diluting substance such as magnesia, there may be formed, at the beginning of the operation, a small amount of magnesium chromite which cannot be made soluble subsequently, this formation taking place according to the reaction.

According to a feature of the present invention, in order to obtain a practically perfect reduction of chrornium oxide at the beginning of the treatment, there is added to the chromium powder a small amount (some thousandths) of magnesium or an equivalent metal in the form of turnings or powder. The term equivalent metal designates a metal giving an oxide the heat of formation of which is higher than that of chromium oxide, such metal being for instance calcium, thorium or zirconium. In the case of magnesium, the reduction of chromium oxide and possibly magnesum chromite takes place according to the following reactions For instance, in the case of the treatment of an electrolytic chromium rich in oxide (that is to say containing an amount of chromium oxide averaging 1%), it suflices to add about 0.5% of magnesium to obtain a perfect elimination of oxygen.

In the particular case where the starting material is a very pure electrolytic chromium in the form of a powder or of relatively big porous lumps, in which the only detrimental impurity seems to be oxygen, it may be possible to limit purification to the removal of oxygen by means of magnesium as above described (the magnesia that is formed being subsequently removed). But the product thus obtained will not be of so high a quality as one obtained by treatment in a halide containing medium.

It should be noted that when the treatment makes use of fluorine compounds, the presence of additional magnesium or of magnesia used as diluting substance might lead to the formation of magnesium fluoride which is diflicult to make soluble subsequently.

In this case, it is preferable to perform the treatment in two steps.

During the first step, oxygen is removed by means of magnesium out of the presence of fluorine compounds, this step being followed by a washing.

The second step includes the action of the fluorine compounds, for instance in the form of a powder of chromium fluoride, this second step preferably taking place without addition of magnesia.

It is to be noted that the addition of magnesium in greater amounts than above mentioned permits the elimination of oxygen from a mixture of chromium powder and chromium oxide.

However, in this case, the amount of magnesium and chromium oxide must not be too high (less than 20% approximately) so as to avoid a reaction which would be too much exothermic. Furthermore, if the removal 6 of oxygen takes place in combination with the action of halides, an excess of magnesium might reduce most of the halides and give rise to the formation of magnesium haildes which are practically inert, which would involve complications in the treatment.

Concerning now nitrogen, it should be noted that the presence of ammonia in the treatment box at tempera tures above 400 C. may have a very detrimental effect because this ammonia would be decomposed and would produce nascent nitrogen.

It should be pointed out that if an ammonium halide is present in the treatment box, it is wholly vaporized at temperatures below 300 C. and any ammonia resulting from its decomposition is eliminated before the temperature reaches 400 C. by the continuous stream of hydrogen which is flowing along the box. In order to be on the safe side concerning the presence of nitrogen in the treatment box, it is advisable to use, as source of halide in said box, a mixture of chromium and iodine. In this case, it will be preferable to use, as halide incorporated in the chromium powder, either chromium iodide (or directly iodine) or chromium fluoride.

The stream of hydrogen circulating along the boxes must not be obtained by cracking ammonia, and the presence of nitrogen in this stream should be avoided as much as possible.

Concerning carbon, it is of course necessary to avoid the introduction of carbon or carbon containing substances such as fatty bodies into the treatment box. It is also advisable to avoid the presence of carbon oxides and hydrocarbons in the stream of hydrogen flowing along the treatment box. And no carbonates and oxalates should be incorporated in the substances acting as sources of halides in the boxes.

Carbon may be eliminated by introducing a small amount of chromium oxide (averaging 1%) in the mass to be treated. For instance a chromium powder to be purified is slightly oxidized, before it is treated, by heating it in the atmosphere at temperatures ranging from 200 C. to 500 C.

Concerning silicon and aluminium, if the treatment takes place in partly gastight boxes as above mentioned, it is advisable, in order to avoid the introduction of said metals into the masses to be treated, to make use of treatment boxes having no parts thereof made of alumina or silica in contact with the atmosphere inside said boxes.

But if the masses are treated directly in a stream of purified hydrogen, that is to say if no treatment boxes are used, it is possible to use an oven having its walls made of alumina.

In order to eliminate silicon and aluminium, the halide to be present in the mass in treatment must be one which forms with silicon or aluminium a halide volatile at the temperature of treatment. For instance if the treatment temperature is 1,000 C., the source of the halide to be formed in the mass in treatment will preferably be iodine because aluminium iodide boils at 300 C. For silicon, the halogen to be used preferably is fluorine.

It it were desired to eliminate iron from the mass in treatment, the halogen to be chosen should be bromine.

Advantageously the prification treatment should be effected under reduced pressure in order to improve the regularity of purification.

To sum up, the various oxides, and even chromium oxide are wholly eliminated by reduction by means of hydrogen in the presence of a halide (and preferably fluoride) atmosphere. Aluminium and silicon are trans formed into volatile halides not very much reducible by hydrogen and which are eliminated from the treatment boxes. Carbon, nitrogen and sulfur are transformed into gaseous compounds which are capable of diffusing from the mass in treatment due to the preliminary elimination of chromium oxide.

It should be noted that, very often, the best results can be obtained with the method according to. this invention by making use, as halides and/or halogens, of combinations thereof, such as:

Chlorine+iodine, or Bromine+chromium fluoride.

The heating of the porous mass in the above mentioned conditions should be performed at temperatures ranging from 600 C. to 1,400 C. As a matter of fact, a temperature of 900 C. is generally necessary to obtain the reduction of chromium oxide. However, in some cases, it may be possible to proceed at temperatures as low as 600 C. On the other hand, at l,400 C. sintering of the chromium grains becomes substantial, thus reducing the porosity of the mass in treatment and preventing the free circulation of gases that is necessary to obtain the desired purification. This maximum admissible temperature may however be lower if the mass to be treated contains, in addition to chromium, another metal which causes sintering to begin at a lower temperature.

According to a feature of the present invention, in order to prevent any sintering of the grains of a chromium or chromium alloy powder during its heating as above mentioned, there is incorporated into said powder, before said heating, an inert diluting substance capable of being eliminated after cooling of the powder subjected to said heating. Elimination of said diluting substance may be performed in any suitable way, for instance by washing, sifting, dissolving, makng use of the difference of density between the diluting substance and the chromium powder.

The inert diluting substance above referred to preferably consists of magnesia because it has the following advantages. The heat of formation of magnesia i very high. It dissolves easily in nitric acid so that there is no difficulty in separating it from the chromium powder after treatment. Magnesium practically does not diffuse in chromium. Finally the magnesium halides are very little volatile.

However the use of other diluting substances, such as lime and baryta, although less advantageous, is not excluded.

A refractory oxide, even if difficult to separate from chromium after the treatment, might also be used if this oxide is one of the elements to be mixed with the chromium powder for subsequent use. The only conditions are that this oxide must have a high heat of formation and that its metal must have practically no tendency to diffuse into chromium.

It is interesting to note that, instead of adding magnesia to a chromium powder, it is possible, according to this invention, diretcly to utilize the mixture of chromium and magnesia powder obtained when preparing chromium in the powder form by a magneso-thermic method.

It should be noted that when magnesia or another inert diluting substance is mixed with the chromium or chromium alloy powder to be treated, this mixture being not pressed, it is possible to carry out the purification treatment above described at temperatures as low as 750 C. and even 600 0., provided that the duration of this treatment is sufficiently long.

The amount of inert diluting substance to be added to the chromium or chromium alloy powder to obtain a ductile chromium powder is the smaller as the temperature of treatment is lower. For temperatures below 900 C., the addition of an inert diluting susbtance may be dispensed with.

However it should be noted that the above indications relate only to the case of very fine chromium powders such as obtained by magneso-thermy in a gaseous phase or by crushing electrolytic chromium. When the dimensions of the grains are greater, the temperature below which the addition of an inert diluting substance is unnecessary becomes higher. By way of indication, when it is merely desired partly to eliminate silicon from a coarse chromium powder obtained by silico-thermy (diameter of the grains greater, as an average, than 0.1 mm.) the addition of an inert diluting substance may be dispensed with even when the treatment is conducted at temperatures ranging from 1200 to 1250 C. It suflices, after treatment, to crush in a mortar the product that is obtained.

By way of example, a chromium powder obtained by crushing electrolytic chromium, and in which the mean grain size was a few hundredths of a millimeter, was mixed with 23% of magnesia, 1% of iodine and 1% of ammonium chloride (by weight). The whole was heated for one hour at 10501100 C. After treatment, the mixture was washed in hot water, then in diluted nitric acid to eliminate magnesia, then again in hot water, and dried.

It is thus possible to obtain chromium powders having very good ductility qualities.

This result is quite remarkable because, up to now, it had been impossible to obtain chromium powders that could be packed to reduce their porosity to values lower than 25% (and in some cases 40%).

The chromium or chromium alloy powders obtained by the method of the present invention, as just above described, can be pressed to such a degree that the porosity is finally reduced to much lower values. For instance a chromium alloy powder which, in its original state, had, when compressed at 5 metric tons per sq. cm., a porosity of 21%, had, after treatment according to this invention, when compressed at the same rate, a porosity of only 9% A chromium powder treated according to the present invention and compressed at 6 metric tons per sq. cm. had a density of 6, which is very high as compared to the density of pure solid chromium, which is 7.

The appended curves of FIG. 3 give the densities of various chromium powders subjected to different pressures of compression.

The pressures of compression, in metric tons per sq. cm., are plotted in abscissas and the densities in ordinates.

Curve I relates to an electrolytic chromium known as Hardy electrolytic chromium, such as it is available commercially. It will be seen that when the pressure of compression is increased from 6 tons to 8 tons the density is but little increased and remains approximately 4.5

Curve II relates to the powder obtained by treating this chromium powder according to the present invention.

It will be seen that, while the density of this treated powder is 2 when no pressure is applied thereto, this density constantly increases when the pressure is increased and when the pressure is 8 tons this density is about 6.6, an extremely remarkable result in view of the fact that the density of pure solid chromium is 7. Thus the density has been multiplied by 3.3 by pressing the powder under a pressure of 8 tons.

Curves III and IV relate respectively to a magnesothermic chromium powder and to a silico-thermic chromium powder, both treated according to this invention. The results are not so good as those obtained with electrolytic chromium powder but they are however very interesting.

The approximative dimensions of the chromium grains are respectively:

For curves I and II, 30 microns; For curve III, 7 microns; and For curve IV, 20 microns.

The point marked P, tabove curve II, corresponds to a product obtained as follows: electrolytic chromium powder was pressed (as indicated by curve 11) to a pressure of 6 tons per sq. cm. Then the block thus obtained was heated for one hour at 1075 C. in a fluoride vapor atmosphere, after which it was subjected to a pressure of 6 metric tons per sq. cm. The density of the product thus obtained was about 6.8, i.e. practically equal to that of pure cast chromium.

While the above indications are concerned with articles obtained from powders in which chromium has been rendered ductile by the method according to this invention while remaining in the state of powder, it should be understood that advantageous results can also be obtained when, still according to this invention, the initial powder has been transformed, by the treatment intended to make chromium ductile, into slightly sintered blocks (hereinafter called half sintered blocks) as above described.

An example is given concerning an electrolytic chromium powder.

This powder was initially pressed at 4 metric tons per sq. cm.

This powder was treated according to the present invention, the heating temperature being 1,150 C., so that the product obtained was in the form of half sintered blocks, the density of which was 4.25.

A block of chromium thus purified was then pressed at 6 metric tons per sq. cm. The density of this final product was 6.2.

Another half sintered block was pressed at only 4 metric tons per sq. cm. and in this case the density of the final product was 5.25.

Another example relates to a 18/8 stainless steel (containing 18% of chromium and 8% of nickel). The initial powder, pressed at tons per sq. cm. had a density equal to 6.6. After treatment according to the invention at 1,l50l,175 C. the purified half sintered product had a density equal to 7. After pressing of this half sintered product at 6 metric tons per sq. cm. the final product obtained had a density of 7.7 (i.e. 97% of the theoretical density, which is 7.93).

After a purification treatment as above described, the masses that have been treated are kept, during the cooling thereof down to a temperature of 300 C., in the atmosphere used for said treatment (fluoride or other halide in the presence of a limited amount of hydrogen in a treatment box) or possibly in an atmosphere of hydrogen of very high purity, in order to prevent the formation of chromium oxide thereon.

Below 300 C., the layer of chromium oxide that could form is negligible.

These masses may then be impregnated with an inert atmosphere such as argon and kept in said atmosphere.

Alternately, the masses having undergone treatment may be impregnated with salts which are relatively little volatile at ordinary temperature, but are volatile at temperatures above 300 C., said salts being in solution in a liquid which is volatile at ordinary temperature. For instance, use may be made of a solution of iron chloride in ethyl alcohol or of a solution of ammonium chloride in methyl alcohol. After evaporation of the solvent liquid, the salt remains in the pores of the mass and it prevents the inflow of air thereinto when the mass is subsequently heated in conditions which would be liable to produce oxidation.

In some'cases, the masses having undergone the purification treatment at a temperature as above specified are then heated to a higher temperature, whil remaining in the same container and in the same atmosphere, until they are changed into blocks which are practically no longer porous, whereby sintering is then achieved. This temperature depends upon the composition of the mass in treatment and it ranges from 1,900" C. to l,200 C. 1,900 C. corresponds to the melting temperature of pure chromium but a temperature ranging from 1,600 C. to 1,700 C. is sutficient to obtain a substantial decrease of sintering capable of preventing a subsequent alteration of the purity of chromium articles thus obtained. If the mass treated includes metals such for instance as nickel,

of agglomeration is increased (ranging from 3 to 5 metric tons per square centimeter instead of 0.5 ton per square centimeter). In other cases, porous masses as above stated which have undergone the purification treatment above described may be sintered or cast in a conventional manner in a subsequent separate operation of heating under a vacuum or in a pure hydrogen atmosphere at temperatures ranging from 1,600 C. to 1,900 C. Whatever be the composition of the mass that has undergone the purification treatment as above described, sintering of said masses at the above indicated temperatures is much easier than if this purification treatment had not taken place.

When the starting material consists of chromium powder, it may be under different forms which come within one of the three following groups:

Alumino-thermic or silico-thermic chromium, Magneso-thermic chromium, and Electrolytic chromium.

Carbon from 0.2 to 0.4%

Nitrogen 0.06%

Oxygen 0.2%

Sulfur 0.02%

Silicon 0.09%

Aluminium 0.10% for silico-thermic chromium and up to 0.8% for alumino-thermic chromium Iron 0.63%

After purification, the proportions of these impurities were as follows:

Nitrogen less than 0.02%

Oxygen from 0.001 to 0.005%

Sulfur less than 0.01%

Silicon practically none Alumina practically none Aluminium less than 0.04%

Iron 0.6% unchanged but not detrimental.

Magneso-thermic chromium is obtained directly in the form of a very fine powder by reducing a very fine commercial powder of chromium oxide (Cr O by means of magnesium. Its purity depends upon that of said chromium oxide.

The proportions of impurities found in some samples were:

Percent Oxygen in the form of C 0 0.5 Nitrogen 0.6 Silicon 0.12 Carbon 0.3

After purification by the method according to this invention, the proportions of Oxygen is less than 0.001%

That of silicon less than 0.01% That of nitrogen less than 0.1% And that of carbon less than 0.1%

Electrolytic chromium is obtained in the porous form by electrolysis of chromic acid. It is not so fine and regular as the preceding one. Its purity depends upon that of the chromic acid from which it is obtained. It always contains an amount of oxygen ranging from 0.3 to 0.5% and a proportion of nitrogen lower than 0.005%.

After purification, the amount of oxygen in the product obtained is undetectable.

In all cases above discussed, purification was obtained as it will be hereinafter described in Example I relative to pure chromium.

The specific conditions in which the operations above described are carried out will be chosen in accordance with the desired result.

If these operations are essentially intended to permit of obtaining a material having a high degree of purity to be used subsequently for the production of refractory alloys, it will not be necessary to operate at very high temperatures. since the purification reaction can take place at temperatures ranging from 600 C. to 900 C. according to the characteristics of the starting material. On the other hand, sintering should be avoided as much as possible.

When the purification operations are carried out at mode-rate temperatures, that is to say at temperatures ranging from 1,000 C. to 1,300 C. it is possible to make use of partly gastight treatment boxes made of iron alloys and to obtain half sintered blocks of pure chromium or of chromium alloys, for subsequent use.

Chromium powder, with chromium fluoride mixed therein, is pressed under moderate pressures approximat ing 0.5 metric ton per sq. cm., a binder (camphor in alcoholic solution) being incorporated in the mass to give it the necessary cohesion. This binder is eliminated subsequently by placing the compressed masses in an oven at 150 C. Camphor dissolved in alcohol may be replaced by iodine in alcohol solution and in this case it is not necessary to pass the masses preliminarily in an oven as above mentioned. For the heating treatment, the masses thus formed are placed in a steel treatment box which contains a suitable amount of chromium fluoride and chromium (obtained by preliminary heating at 300 C. of a mixture of chromium and ammonium fluoride). However, preferably the source of halide in the treatment box is constituted by a mixture of chromium and iodine. The half sintered blocks of chromium or chromium alloy obtained after the heating operation may be used (although they have a porosity which may range from 30 to 35%) as elements for making, under a vacuum, refractory alloys, for instance of the Nimonic type, which necessitate a purified chromium or chromium alloy.

The invention makes it possible to operate in the same manner to obtain chromium alloys which contain nickel. In this case, since nickel has a substantial ductility the incorporation of a binder such as camphor may be dispensed with. The pressure at which the material is initially pressed may then reach 1 or 2 metric tons per sq. cm. The temperature of heating ranges from 1,000 to 1,100 C. Subsequently the temperature may be raised to 1,200 C. to reduce the porosity after the purification operation so that no oxidation can take place before the blocks are used.

The mixture of chromium powder and chromium fluo ride, which may also contain at least one powder of another metal such as nickel, may also be treated in an open retort made of a refractory oxide such as zirconium oxide, placed in a hydrogen stream, the mixture of powders being not initially pressed. In this case, at the end of the treatment, it is advisable to eliminate the superficial layers of the articles obtained which may be slightly too rich in zirconium oxide or may have sintered with said oxide. A purification operation such as above described may be followed by a sintering operation, merely by in creasing the temperature and/ or the duration of the treatment.

The sintering operation which may follow the purification operation, in the same treatment box, may also be performed separately and in this case, after a purification as above described, sintering is performed in conventional apparatus, in a vacuum or in an atmosphere of highly purified hydrogen.

If, after a chromiumagglom'erate or powder has been purified by treatment at'temperatures ranging from 1,000 C. to l,300 C., the temperature is raised up to a temperature ranging from 1,400 C. to 1,800 C. in a treatment box at least the inner wall of which is made of molybdenum and complete sintering is achieved, it gives pieces without porosity and relatively ductile. The articles may have been initially given the desired shape, for instance by moulding of a mixture containing a binder. Alternately chromium masses may be given a simple geometrical shape such as a parallelepipedal one, and after the purification treatment the blocks thus obtained are machined, for instance by spark machining. Of course, what has been said concerning the treatment of pure chromium remains true when it is desired to treat chromium alloys containing iron, nickel, cobalt, molybdenum, tungsten and titanium for instance. The only difl'erence is that in this case the sintering temperature may be lower.

Chromium purification may also be effected when chromium in the powder form has been mixed with a powder of another material.

For all these treatments, the conditions of the purification operations have been above described, the temperature being chosen, as above stated, within a range extending from 600 C. to 1,400 C.

Various examples of the invention will now be given:

Example I This example is concerned with the treatment of an electrolytic chromium powder the grains of which have dimensions ranging from one hundredth to three hundredths of a millimeter, this type of chromium containing no dangerous impurities other than oxygen, the percentage of which is 0.46. The chromium powder is mixed with 2% of pure chromium fluoride in the state of a very fine powder. This mixture is agglomerated by means of a binder constituted by iodine dissolved in alcohol so as to form blocks which are pressed under a pressure of 0.5 ton per sq. cm. These blocks are placed in partly gastight nickel boxes heated in an atmosphere of hydrogen. Thus the atmosphere inside the boxes consists of chromium fluoride vapors in the presence of hydrogen. The heating is effected at a temperature of 1,200 C. for 6 hours. At the end of the operation, the half sintered pieces that are obtained have a porosity of 25% and a substantial plasticity. Said pieces contain practically no trace of oxygen.

Example 11 This example relates to the treatment of magnesothermic chromium. This chromium is mixed with chromium fluoride in the proportion of 99% of chromium for 1% of chromium fluoride by Weight. The mixture of powder is agglomerated, under a pressure of 2 metric tons per sq. cm., by means of a few percents of camphored alcohol so as to form blocks which are heated for some time at C. to drive oif said camphored alcohol. These blocks are heated for 6 hours at 1,250l,300 C. in an alumina container through which a stream of hydrogen circulates at a low flow rate (a few cubic centimeters per second). This hydrogen has been carefully purified by passing it over platinized asbestos, drying it on phosphoric anhydride (P 0 and passing it over titanium oxide in the spongy state at 900 C.

Half sintered blocks pure chromium are thus obtained. These blocks are coated with a bright film of chromium due to the reduction of a portion of the chromium halide on the surface of the blocks. In order to limit the reduction of the chromium fluoride incorporated in the powder, chromium saturated with chromium fluoride may be placed in the oven, upstreamof the agglomerates to be purified. In other cases, the hydrogen stream circulating through the oven was made to pass through another oven containing chromium and chromium fluoride.

13 Example III This example is concerned with the obtainment of a material to be used to form a chromium nickel alloy. Use is made of a mixture of magneso-thermic chromium (49%), in which the only impurity is oxygen (0.6%), of nickel (49%) and of chromium fluoride. Blocks were formed by pressing at 1 metric ton per sq. cm. and these blocks were heated in partly gastight treatment boxes where a mixture of iodine and chromium powder corresponding to a few percents of the weight of the treated mass had been placed. These boxes were subjected to a temperature of 1,1001,150 C. for 3 hours in a stream of hydrogen.

After this, the temperature was raised to l,200 C. so as to reduce the porosity to a few percents. Then, the blocks were allowed to cool down in a protective atmosphere of pure hydrogen. The small blocks of nickel chromium alloy thus obtained were used for the production in a vacuum of nickel alloys.

Example IV This example is concerned with refractory materials made of chromium alone.

Use was made of a mixture of finely powdered electrolytic chromium (containing only 0.5% of oxygen) with 1% of chromium fluoride and 1% of camphor in the form of camphored alcohol.

This powder was pressed at a pressure ranging from 0.5 to 4 tons per sq. cm. to form a block which was heated for 2 hours at 150 C. to eliminate camphor.

This block was heated in a partly gastight treatment box containing a mixture of chromium and chromium fluoride, for 2 hours at 1,000 C., said box being surrounded by a stream of hydrogen.

Subsequently the box was heated to a temperature of 1,700 C. for 3 hours.

The box was provided with a protective screen of chromium as shown at 8 on FIGS. 1 and 2.

The block of sintered chromium thus obtained had remarkable qualities:

It contained practically no oxygen,

It was ductile and could be forged at temperatures ranging from 50 to 100 C. without risk of breaking,

It could be marked by means of a punch,

It could be milled and drilled without special precautions, and

It could be polished electrically.

According to a modification, the ductility of the articles finally obtained was improved by incorporating in the mixture of powders to be treated 1% by weight of cerium in the form of cerium fluoride. It is believed that this cerium fixes, in the form of nitrides which do not enter the chromium network, the last traces of nitrogen which may remain in the material.

Example V This example relates to the production of chromium plates of high resistance and density capable of being used in a nuclear reactor.

.A fine powder of electrolytic chromium (containing 0.5% of oxygen) is mixed intimately with 1% of magnesium in the form of turnings, 0.2% of zirconium, of magnesia, 1% of iodine and 1% of bromine. The whole is heated for 1 hour at a temperature of 1,125-1,150 C. in treatment boxes made of iron (and the inner surface of which is already chrornized), these boxes being only partly gastight and located in a stream of ordinary hydrogen. After treatment and separation of the magnesia by means of nitric solutions, the plasticpowder that is obtained is given the shape of plates by pressing at 6 metric tons per sq. cm. The density of the plates thus obtained is higher than 6.2. These articles are sintered for two hours at a temperature ranging from 1,700 C. to 1,750 C. in boxes containing magnesia powder acting as an inert support for said plates. On the other hand,

magnesia had been added, in the region of the retort where the temperature is minimum, together with 1% of chromium fluoride (obtainedby heating a mixture of chromium powder in ammonium fluoride). A small amount of chromium had been intimately mixed with magnesia. The whole was heated in an atmosphere of ordinary hydrogen. Such plates had a bending strength, at ordinary temperature, of 35 kilograms per sq. cm.

The plates obtained by pressing the plastic chromium powder at 6 metric tons per sq. cm., instead of being subsequently heated at 1,7001,750 C., have been, according to a modification, heated at 1,100" C. In this case, their bending strength, at ordinary temperature, was 26 kilograms per sq. cm.

Example VI This example relates to a mere removal of silicon from a silico-thermic chromium, in order to obtain a material for use in the preparation of cobalt alloys.

This chromium in the state of a powder the grains of which have dimensions ranging from 0.1 to 0.3 mm. is mixed with 2% of chromium fluoride in the state of a fine powder, the whole being heated for 2 hours at 1,150-1,200 C, in treatment boxes as above described, in an atmosphere of hydrogen.

Example VII This example relates to the manufacture of filters from a powder of a composition corresponding to stainless steel and in which the dimensions of the grains range from to 200 microns. This powder was pressed at 1 ton per sq. cm. and was heated for 2 hours at 1,180" C. in the presence of a source of chromium fluoride constituted by a mixture of chromium and ammonium fluoride. This treatment took place in iron boxes, of the type above referred to, placed in a stream of hydrogen.

Example VIII This example also relates to the manufacture of pieces made of stainless steel.

'It was found, according to this invention, that the behaviour of such pieces obtained from alloyed powders treated as above mentioned may be further improved if, during the treatment serving to the purification of the alloy, the grains of the powder are superficially enriched in chromium owing to the addition to the powder in treat.- ment of a small amount of chromium powder and/or of a chromium halide capable of conveying, by means of its vapors, chromium to the surface of the grains during the heating treatment.

The present example relates to the obtainment of filters for corrosive liquids (such as acetic acid) from a powder having a composition corresponding to that of 18/8 stainless steel (that is to say a stainless steel containing 18% of chromium and 8% of nickel). The grains of this powder mixture had dimensions ranging from 50 to microns.

This powder was mixed intimately with 3% of a very fine powder of chromium and 1% of chromium fluoride (the mixture of chromium powder and chromium fluoride had been obtained in a preliminary operation by heating a mixture of chromium powder and ammonium fluoride),

This powder was pressed at 0.5 metric ton per sq. cm. to form articles which were heated for 2 hours at 1,250- 1,275 C., in partly gastight treatment boxes where they were supported by plates of sintered chromium and which contained a mixture of chromium and ammonium fluoride to obtain a supplementary enrichment in chromium of the surface of the grains.

These boxes were heated in a furnace in an atmosphere of cracked ammonia (the presence of nitrogen having no detrimental effect in the case of stainless steel pieces). Cooling of the boxes was effected as quickly as possible.

The same conditions of treatment were applied for the obtainment of mechanical pieces but in this case the starting material was a powder the grains of which were of smaller dimensions (some hundredths of a millimeter of diameter) and the pieces were obtained by pressing said powder at pressures averaging metric tons per sq. cm. The pieces thus obtained were practically non-porous and entirely inoxidizable.

Example IX This example relates to the manufacture of filters by means of a mixture of iron powder having grains the dimensions of which range from 300 to 500 microns, of very fine chromium and of 2% of chromium in the form of chromium fluoride.

This powder was pressed at a pressure of 2 metric tons per sq. cm. to obtain agglomerates which were heated for 2 hours at 1,200 C.

Filters obtained in this way have a perfect behaviour in salt water and nitric acid. Micrography showed that the surface of these grains is enriched in chromium so as to contain 40% thereof.

It should be noted that analogous but not quite so good results were obtained by adding to the iron powder 1% of iodine and 17% of chromium or again 2% of chromium in the form of chromium chloride and 35% of ferrochromium powder containing 65% of chromium.

For the manufacture of filters, other halides than the fluoride, and for instance iodines or chlorides, may also be used.

Example X This example is concerned with the manufacture of mechanical pieces capable of resisting oxidation and made from a mixture of iron powder of a. grain size of about microns, of 19% of chromium in the state of a fine powder of a grain size of some hundredths of a millimeter and of 1% of chromium in the form of chromium fluoride.

This mixture of powder is compressed to 4 metric tons per sq. cm.

The block obtained after this present operation was heated for one hour at 900 C. in the presence of a mixture of chromium and ammonium fluoride. Then, the temperature was raised to 1,225l,250 C. for 3 hours after sweeping by small amounts of ammonium fluoride. The pieces thus obtained were entirely inoxidizable and capable of resisting the action of salt water and nitric acid. Said pieces were practically non-porous, capable of undergoing bending and tensile deformations. Their Vickers hardness was 120. They registered dry oxidation at temperatures of above 900 C. without cracking corrosion.

These results are the more remarkable as said pieces do not contain any trace of nickel.

Equivalent results were obtained with materials of lower cost by making use of a mixture of iron powder of a grain size ranging from 20 to 50 microns and of of a ferro-chromium powder containing 65% of chromium. After preliminary heating with ammonium fluoride so as to transform 2% of the ferro-chromium powder into a mixture of chromium fluoride and iron fluoride, the powder was pressed at 5 metric tons per sq. cm., and the purifying heating and the sintering heating were performed in the same conditions as above stated.

Example XI This example is concerned with alloys of chromium and cobalt.

Use was made of a mixture of chromium powder (30%) (1% being in the form of chromium fluoride), molybdenum (8%) and cobalt (62%), all these powders having grain dimension of one hundredth of a millimeter.

The powder was pressed at pressures ranging from 5 to 6 metric tons per sq. cm.

The mixture was first subjected to a purification heating of 1 hour at 1,050 C. in the presence of a little volatile fluoride (for instance cobalt or nickel fluoride). Then the material was sintered by heating for 6 hours at l,250 C. in a chromizing atmosphere (chromium fluoride vapors) which increased the resistance of the final product to oxidation at high temperatures and made it possible to improve the resistance at temperatures above l,l00 C. This c-hromizing heating was performed in the presence of a mass of chromium placed in the vicinity of the blocks in treatment, said chromium being held by grids of a molybdenum wire.

This last mentioned chromium made it possible to regenerate the chromium fluoride vapors by the action thereon of hydrofluoric acid formed as a result of the chromizing operation.

This regeneration chromium could also be introduced in the form of sintered chromium surrounding the article in treatment and preferably separated therefrom by molybdenum or tungsten wires.

Example XII This example relates to the obtainment of pieces containing chromium and molybdenum.

Molybdenum is simultaneously sintered and chromized.

At a matter of fact, is is necessary to eflect sintering and chromizing of molybdenum simultaneously because if the powder having already undergone a chromizing was sintered, a diffusion of chromium to the core of the grains would take place at 1,5001,600 C. and would produce an excessive brittleness.

Use is made of a mixture of 70% of molybdenum in the form of relatively big grains of a size ranging from 50 to 100 microns, of 30% of chromium (2% of which are in the form of chromium fluoride). The powder is pressed at 3 metric tons per sq. cm.

The compressed pieces are first heated for 2 hours at 1,250 C. in the presence of iron fluoride, after which sintering proper takes place, simultaneously with chromizing, by heating at 1,550-600 C. for 3 hours.

The bodies have resistance to oxidation up to 1,l00 C., which is improved with respect to the resistance of conventional alloys. Their brittleness is lower than that of said conventional alloys sintered at temperatures ranging from l,600 C. to 1,700 C.

Example XIII A mixture of magneso-thermic chromium with 40% of magnesia (such a mixture being that obtained as an intermediate product during the manufacture of magnesothermic chromium) is added to 1.5 percent of bromine (bromine being poured into the bottom of a treatment box and the mixture of chromium and magnesia being poured thereon).

The treatment box is only partly gastight and is located in a hydrogen atmosphere.

The whole is heated for 2 hours at 1,050" C.

The product thus obtained is a plastic silver white powder (the initial powder was black).

Example XIV This example relates to the treatment of a mixture of iron, nickel and chromium powders mixed together in proportions corresponding to a 18/8 stainless steel, the diameter of the grains ranging from 50 to 80 microns.

of this powder was mixed with 23% of magnesia, 1% of magnesium and 1% of iodine.

The whole was placed in a treatment box as above.

referred to, placed in a hydrogen atmosphere, and heated for 30 minutes at l,080 C.

What I claim is:

1. The method of purifying chromium material comprising heating a mass of juxtaposed grains of said material in a porous state at a temperature above 600 C., said material being selected from a group consisting of chromium and chromium alloys, said grains having an average size below 0.1 mm, said grains being surrounded constantly during said heating by vapors of a halide of a metal contained in said material and an amount of 75 hydrogen sufliciently small to avoid reduction of all of said metal halide, the length and temperature of said heating being such to achieve said chromium purification before any substantial sintering of said material has occurred, and protecting said material during said heating and while cooling against contact with any material amounts of any of the gases selected from the group consisting of oxygen, water vapor, nitrogen and ammo- 2. The method of claim 1 wherein said metal halide in a non-gaseous state is contained in said material, said material is placed in an open container, and said container is placed in a stream of purified hydrogen during said heating.

3. The method of claim 1 in which said heating is conducted for at least 30 minutes at temperatures ranging from 600 C. to 1,400 C.

4. The method of claim 1 in which said heating is conducted for at least one hour at temperatures ranging from 900 C. to 1,300 C.

5. The method of claim 1 wherein said mass also includes a metal of the group consisting of magnesium, calcium, thorium and zirconium.

6. The method of claim 1 wherein said mass also includes less than 1% of magnesium whereby magnesia is formed with chromium oxide impurities in said mass, and removing said magnesia therefrom.

7. The method of claim 1 which additionally comprises adding to said material before said heating, a second metal having a heat of formation of its oxide greater than the heat of formation of chromium oxide.

8. The method of claim 7 wherein said second metal is magnesium.

9. The method of purifying chromium material comprising placing a mass of juxtaposed grains of said material in a porous state in a container having a partially gas-tight closure, said material being selected from a group consisting of chromium and chromium alloys, said grains having an average size below 0.1 mm., placing in said container a substance capable on heating of producing vapors of a halide of a metal contained in said material, maintaining said container in a hydrogen atmosphere, and heating said container above 600 C. at temperature and time to achieve said chromium purification before any substantial sintering of said material has occurred.

10. The method of claim 9 which further comprises placing near said closure in said container a screen of chromium to stop oxygen impurities that may exist in the hydrogen atmosphere surrounding said container.

11. The method of claim 9 wherein said substance capable of producing vapors is said metal halide.

12. The method of claim 9* wherein said substance capable of producing vapors is a halogen which reacts with said metal.

13. The method of claim 9- which additionally comprises prior to said heating above 600 C. the formation of chromium oxide on said material by an earlier heating 18 step in the atmosphere at a temperature range from 200 to 500 C. for the elimination of carbon.

14. The method of claim 9 which prior to said placement of said mass in said container additionally comprises agglomerating said grains with a binder and subsequently removing said binder by an earlier heating step at a temperature range from 200 to 300 C.

15. The method of purifying chromium material selected from the group consisting of chromium and chromium alloys comprising heating a mass containing powder of said material at a temperature range between 600 and 1400 C., said material powder being mixed with a metallic oxide powder having a heat of formation higher than chromium oxide and the metal of which does not diffuse into chromium, the grains of said mass being surrounded constantly during said heating by vapors of a halide of a metal contained in said mass and an amount of hydrogen sufficiently small to avoid reduction of all of said metal halide, and protecting said mass during said heating and while cooling again contact with any material amounts of any of the gases selected from the group consisting of oxygen, water vapor, nitrogen and ammonia.

16. The method of claim 15 in which said metallic oxide is eliminated from the product contained after the heating step.

17. The method of claim 15 in which said metallic oxide remains in the finished product.

18. The method of claim 15 wherein said metallic oxide powder is magnesia.

19. The method of claim 18 in which the mixture of chromium powder and magnesia powder is constituted by the intermediate product obtained during the manufacture of chromium powder by the magneso-thermic method.

20. The method of claim 15 which further comprises the step of removing said magnesia from the product obtained after the heating step by treating said product with nitric acid.

References Cited in the file of this patent UNITED STATES PATENTS 1,878,589

OTHER REFERENCES Sully: Chromium, Metallurgy of the Rarer Elemen Series, publ. 1954, pages 55-57.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,053,649 September 11, 1962 Philippe Galmiche It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3, line 36, for "containing" read contain same column 3, line 75, and column 41., line 1, for "Kantahl" read Kanthal column 5, line 37, for "3Mg2 Cr" read 3Mg-- 2Cr column 6, line 4, for "haildes" read halides line 61, for "prification" read purification column 7, line 29', for "makng" read making line 51, for "diretcly" read directly line 66, for "suslotance" read substance column 10, line 58, for "C 0 read 01- 0 column 16, line 22, for .ffAt" read As same line 22, for "is", first occurrence, read it line 36, for "1,550-600 C. read 1,5501,600 l n 18, Line 20, for "again" read against Signed and sealed this 9th day of April 1963.,

.SEAL) \ttest:

ZSTON G. JOHNSON DAVID L, A ,ttesting Officer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,053,649 September 11, 1962 Philippe Galmiche It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3, line 36, for "containing" read contain same column 3, line 75, and column 4, line 1, for "Kantahl" read Kanthal column 5, line 37, for "3Mg2-9Cr" read 3Mg-- 2Cr column 6, line 4, for "haildes" read halides line 61, for "prification" read purification column 7, line 29', for "makng" read -'making line 51, for "diretcly" read directly line 66, for "susbtance" read substance column 10, line 58, for "C 0 read Cr column 16, line 22, for .ffAt" read As same line 22, for "is", first occurrence, read it line 36,

for "1,550600 C-" read l,550-l,600 (1 column 18, line 20, for "again," read against Signed and sealed this 9th day of April 1963.

(SEAL) Attest:

ESTON G. JOHNSON DAVID L. LADD Attesting Officer Commissioner of Patents 

1. THE METHOD OF PIRIFYING CHORMIUM METERIAL COMPRISING HEATING A MASS OF JUXTAPOSED GRAINS OF SAID MATERIAL IN A POROUS STATE AT A TEMPERATURE ABOVE 600* C., SAID MATERIAL BEING SELECTED FROM GROUP CONSISTING OF CHROMIUM AND CHORMIMUM ALLOYS, SAID GRAINS HAVING AN AVERAGE SIZE BELOW 0.0 MM,. SAID GRAINS BEING SURROUNDED CONSTANTLY DURING SAID HEATING BY VAPORS OF A HALIDE OF A METAL CONTAINED IN SAID MATERIAL AND AN AMOUNT OF HYDROGEN SUFFICIENTLY SMALL TO AVOID REDUCTION OF ALL OF SAID METAL HALID, THE LENGTH AND TEMPERATURE OF SAID HEATING BEING SUCH TO ACHIEVE SAID CHROMIUM PURIFICATION BEFORE ANY SUBSTANTIAL SINTERING OF SAID MATERIAL HAS OCCURED, AND PROTECTING SAID MATERIAL DURING SAID HEATING AND WHILE COOLING AGAINST CONTACT WITH ANY MATERIAL AMOUNTS OF ANY OF THE GASES SELECTED FROM THE GROUP CONSISTING OF OXYGEN, WATER VAPOR, NITROGEN AND AMMONIA. 